I think you're looking for IORef http://www.haskell.org/ghc/docs/
latest/html/libraries/base/Data-IORef.html
Something like this (untested) should do what you want:
example :: IO ()
example = do { ref <- newIORef 1000; loop ref }
where loop ref = do
x <- readIORef ref
print x
when (x>1) (writeIORef ref (x/2) >> loop ref)
On Feb 2, 2006, at 10:57 AM, Maurício wrote:
> Donald Bruce Stewart wrote:
>> briqueabraque:
>>> Hi,
>>>>>> I would like to know what options I have in Haskell to do
>>> something similar to this C++ code:
>>>>>> double a = 1000;
>>> while (a>1) a/=2;
>>>>>> I'm able to do that with lists, but I would like to know how to
>>> do that with monads and variables with state.
>> You'll get good code using a normal recusive loop:
>> main = print (loop 1000)
>> where
>> loop a | a <= 1 = a | otherwise
>> = loop (a/2)
>> All such control structures may be implemented using recursion.
>> -- Don
>> I understand those examples, but I really would like to know how
> to do that with monads. I would like to ask the same question, but
> now with this code:
>> double a = 1000;
> double b = 0;
> while (a != b) {
> a /= 2;
> cout << a; // Prints a
> cin << b; // User gives a number, stored in b
> };
>> Best,
> Maurício
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